The ImportedFunctionEntry and IndirectFunctionTableEntry stored handles
internally, but were created from raw pointers. This is not allowed.
The two options to fix this are to either handlify the whole interface,
or do the opposite and use raw pointers everywhere. Since no current
user depends on a handlified interface, and both objects are being used
in performance critical code, this CL unhandlifies the interface and
adds a DisallowHeapAllocation scope to enforce that no GC happens while
any ImportedFunctionEntry or IndirectFunctionTableEntry is alive.

R=mstarzinger@chromium.org
CC=titzer@chromium.org

Change-Id: I098c2abcdd28c4b117272ac3ea0358ff2e56b36c
Reviewed-on: https://chromium-review.googlesource.com/1005075
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52531}